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NullReferenceException when streaming XML asynchronously from DB #1903
Description
In our environment we frequently stream XML columns from the database using a pattern similar to the one in the repro test case.
After upgrading to Microsoft.Data.SqlClient 5.1.0, we're getting NullReferenceExceptions for code that was previously working.
Exception message: An exception of type 'System.NullReferenceException' occurred in System.Private.CoreLib.dll but was not handled in user code: 'Object reference not set to an instance of an object.'
Stack trace:
at Microsoft.Data.SqlClient.SqlDataReader.SqlDataReaderAsyncCallContext`2.DisposeCore()
at Microsoft.Data.SqlClient.AAsyncBaseCallContext`2.ClearCore()
at Microsoft.Data.SqlClient.AAsyncBaseCallContext`2.Dispose()
at Microsoft.Data.SqlClient.SqlDataReader.CompleteAsyncCall[T](Task`1 task, SqlDataReaderBaseAsyncCallContext`1 context)
at Microsoft.Data.SqlClient.SqlDataReader.InvokeAsyncCall[T](SqlDataReaderBaseAsyncCallContext`1 context)
--- End of stack trace from previous location ---
at System.Runtime.CompilerServices.ConfiguredValueTaskAwaitable`1.ConfiguredValueTaskAwaiter.GetResult()
at System.Xml.XmlTextReaderImpl.<ReadDataAsync>d__504.MoveNext()
at System.Xml.XmlTextReaderImpl.<FinishInitTextReaderAsync>d__477.MoveNext()
at System.Xml.AsyncHelper.<CallBoolTaskFuncWhenFinishCoreAsync>d__11`1.MoveNext()
at System.Xml.Linq.XDocument.<LoadAsyncInternal>d__25.MoveNext()
at Program.<<Main>$>d__0.MoveNext() in C:\Users\user\source\repos\testsql\Program.cs:line 43
at Program.<<Main>$>d__0.MoveNext() in C:\Users\user\source\repos\testsql\Program.cs:line 33
at Program.<<Main>$>d__0.MoveNext() in C:\Users\user\source\repos\testsql\Program.cs:line 33
at Program.<<Main>$>d__0.MoveNext() in C:\Users\user\source\repos\testsql\Program.cs:line 33
To reproduce
This is the simplest test case I could build for the error we're seeing. It's the same:
- on-prem or Azure SQL
- Windows or Linux
- .NET 6.0 or 7.0
If you downgrade SqlClient to 5.0.1 you will get the expected result; otherwise there's a NullReferenceException. Applying one of the HACK: lines below will usually provide the expected result with 5.1.0, but not always. (e.g. We've seen process hangs after using the IsDBNullAsync workaround.)
using System;
using System.Data;
using System.Xml;
using System.Xml.Linq;
using Microsoft.Data.SqlClient;
// configurations tested with identical results:
// -Azure SQL with AAD MSI on Windows (IIS/ASP.NET 6.0)
// -Azure SQL with AAD Interactive on Windows (.NET 7.0)
// -Azure SQL with SQL username/password on WSL2 Ubuntu Jammy (.NET 7.0)
// -on-prem SQL 2017 CU 31 with Trusted Connection on Windows (.NET 7.0)
const string ConnectionString = "<provide your own>";
/*
* The code listed below will throw an NullReferenceException with SqlClient 5.1.0.
* If you change the SqlClient package to 5.0.1, or apply one of the three HACK lines
* of code below you will get the expected output.
*/
await using var con = new SqlConnection(ConnectionString);
await using var command = new SqlCommand(
// original case was selecting a single row from a table with an XML NOT NULL column; same behavior observed
"SELECT CAST('<test>This is a test string</test>' AS NVARCHAR(MAX))",
con
) { CommandType = CommandType.Text };
await con.OpenAsync().ConfigureAwait(false);
await using var reader = await command.ExecuteReaderAsync(
// HACK: no NRE if you comment out SequentialAccess
CommandBehavior.SequentialAccess
).ConfigureAwait(false);
var xmlOpts = new XmlReaderSettings() { Async = true };
while (await reader.ReadAsync().ConfigureAwait(false)
// HACK: no NRE if you check for DBNull, even though we know it's not null
//&& !await reader.IsDBNullAsync(0).ConfigureAwait(false)
)
{
using var textR = reader.GetTextReader(0);
using var xmlR = XmlReader.Create(textR, xmlOpts);
// HACK: no NRE if you load xmlR synchronously
//var xdoc = XDocument.Load(xmlR);
var xdoc = await XDocument.LoadAsync(xmlR, LoadOptions.None, default)
.ConfigureAwait(false); // NRE thrown here
// Expected: "<test>This is a test string</test>"
Console.WriteLine(xdoc.ToString());
}Expected behavior
Prints the test XML string to the console.
Further technical details
Microsoft.Data.SqlClient version: 5.1.0
.NET target: .NET 6.0, .NET 7.0
SQL Server version: Azure SQL, SQL Server 2017
Operating system: Windows 2019, Windows 10 21H2, Ubuntu 22.04
Activity
lcheunglci commentedon Jan 25, 2023
Hi @NickKerensky, thanks for reporting this issue and providing the sample repro code snippet. It sounds like a regression that might be caused by one of our changes in the release. We'll do some investigation and get back to you soon.
Wraith2 commentedon Jan 25, 2023
From the stack trace this looks like code I've been very involved with. Do you want me to have a look @lcheunglci ?
lcheunglci commentedon Jan 25, 2023
Thanks @Wraith2, your help would be much appreciated. :)
Wraith2 commentedon Jan 25, 2023
Simple fix, just need to add a
?in the right place. #1906lcheunglci commentedon Jan 26, 2023
Thanks @Wraith2!
udlose commentedon Feb 21, 2023
@Wraith2 , @lcheunglci Anyone have an idea when a hotfix will be released? We are currently experiencing the same NRE. Is it possible to have something published before 5.2.0?
Wraith2 commentedon Feb 22, 2023
No idea. you could use the artifacts from the PR if you want.
lcheunglci commentedon Feb 22, 2023
Hi @udlose, since MDS 5.1 is an LTS release, we would expect a hotfix release with this fix included in the near future. However, I don't have the exact date at the moment as we're in the middle of planning for the first half of the year. Once we have a concrete date, we'll let you know, but as @Wraith2 mentioned, you can use the artifacts from the PR at the moment.
lcheunglci commentedon Feb 22, 2023
We currently don't have a schedule for a 5.1.1 hotfix release; however, depending on the number of bugs and severity, it could happen within a month or 2.
lcheunglci commentedon Mar 29, 2023
@udlose the 5.1.1 hotfix release came out yesterday that include this fix .